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Photometry groups

Astronomical catalogues often contain information about photometric measures of the catalogued objects (usually as magnitudes). When this happens it is very useful to have as much information about these magnitudes as possible.

The IVOA Photometry Data Model specifies how to describe a photometric value (and/or the associated photometric filter) in the VO context and you could want to read it for further information.

In the configuration you must do two different things:

• In the "Fields" tab (see figure above), group together those columns that correspond to the same "photometric filter" (for instance, a magnitude and its error will be in the same group). That means:
  • For each magnitude, give an arbitrary name to the "Phot.Group" column (usually, a different one for each magnitude).
  • It there are other columns that can be associated to a given magnitude, repeat the same "Phot.Group" name that you chose for the magnitude.
  • In our example catalogue we have four different magnitudes and we have chosen:
    • magr field ⇒ sdss_r group.
    • sig_magr field ⇒ sdss_r group (this corresponds to the same "filter" than magr so we assign the same group to it and, thus, they will share the same filter information.
    • magJ field ⇒ 2mass_j group.
    • magH field ⇒ 2mass_h group.
    • magKs field ⇒ 2mass_ks group.
• In the "Photom. Groups" tab:
  • give the main characteristics of each photometric filter.

For each photometry group you just need to specify:

• 'Phot.Group': the name for the photometry group.
• 'Value type': if it is a magnitude or a flux (flux, flux density.. whatever).
• 'Filter ID': The filter name in the SVO FPS.
• 'Mag type': Is this a pogson magnitude (the usual case) or asinh o linear?
• 'Zero Point': if the value is given as a magnitude and you do not find the right filter in the Filter Profile Service, you can give a zero point here (in erg/cm2/s/A, please).
• 'ZP type': Is the zero point in Vega, AB or ST system?

We assume here that each photometric value can be described as corresponding to one of the photometry filters available in the SVO Filter Profile Service. Ideally, this means that the observation has been made using that filter, but it is enough if you think that the observing filter is close enough to one in the Filter Profile Service.

Giving a Filter ID for the observation implies including a lot of information about the filter and, thus, about the observation (and how to understand a given magnitude/flux).

All this information about filters will be used in the web page so that when a user moves the mouse over the table header a small windows appears giving the description of each field.

And, what's more important, it will be used in the VO ConeSearch response to include metadata groups with the fields corresponding to the same filter and the appropriate links to the Filter Profile Service's to get extra information about the filter and the adequate calibration. This allows a VO application to automatically retrieve all the data that it needs about this photometric measure. It could be a zero point to transform it to flux, a wavelength to plot the value or even the actual filter transmission curve to compute synthetic photometry to compare with the observed one.

... <GROUP name="phot_Jmag"> <PARAM name="phot_Jmag_fid" utype="photDM:PhotometryFilter.uniqueIdentifier" datatype="char" arraysize="*" value="2MASS/2MASS.J"/> <PARAM name="phot_Jmag_filterurl_web" datatype="char" arraysize="*" value="http://svo2.cab.inta-csic.es/theory/fps/index.php?id=2MASS/2MASS.J"/> <PARAM name="phot_Jmag_filterurl_vot" datatype="char" arraysize="*" value="http://svo2.cab.inta-csic.es/theory/fps/fps.php?ID=2MASS/2MASS.J"/> <PARAM name="phot_Jmag_pcid" utype="photDM:PhotCal.identifier" datatype="char" arraysize="*" value="2MASS/2MASS.J/Vega"/> <PARAM name="phot_Jmag_pcurl_vot" datatype="char" arraysize="*" value="http://svo2.cab.inta-csic.es/theory/fps/fps.php?PhotCalID=2MASS/2MASS.J/Vega"/> <PARAM name="phot_Jmag_mtype" utype="photDM:PhotCal.magnitudeSystem.type" datatype="char" arraysize="*" value="pogson"/> <FIELDref ref="jmag"/> </GROUP> <GROUP name="phot_Hmag"> <PARAM name="phot_Hmag_fid" utype="photDM:PhotometryFilter.uniqueIdentifier" datatype="char" arraysize="*" value="2MASS/2MASS.H"/> <PARAM name="phot_Hmag_filterurl_web" datatype="char" arraysize="*" value="http://svo2.cab.inta-csic.es/theory/fps/index.php?id=2MASS/2MASS.H"/> <PARAM name="phot_Hmag_filterurl_vot" datatype="char" arraysize="*" value="http://svo2.cab.inta-csic.es/theory/fps/fps.php?ID=2MASS/2MASS.H"/> <PARAM name="phot_Hmag_pcid" utype="photDM:PhotCal.identifier" datatype="char" arraysize="*" value="2MASS/2MASS.H/Vega"/> <PARAM name="phot_Hmag_pcurl_vot" datatype="char" arraysize="*" value="http://svo2.cab.inta-csic.es/theory/fps/fps.php?PhotCalID=2MASS/2MASS.H/Vega"/> <PARAM name="phot_Hmag_mtype" utype="photDM:PhotCal.magnitudeSystem.type" datatype="char" arraysize="*" value="pogson"/> <FIELDref ref="hmag"/> </GROUP> ...

You don't need to find that information very intuitive. It's there more for being understood by applications than for human reading. Just as a short summary, in each GROUP there are several PARAM's giving information about the group (in this case, links to the Filter Profile Service filter and calibration) and one or several FIELDref's giving the fields that share those properties.